The present invention relates to a spring element which is composed of an elastic material, in particular of synthetic plastic, such as for example polyurethane, for receiving kinetic energy.
In addition to the classic springs of steel (such as helical springs, friction springs) and the hydraulic springs (shock absorbers), springs of synthetic plastic for receiving higher kinetic energies are also popular. A certain disadvantage of springs of synthetic plastic is that, their properties change in response to high temperature fluctuations. This disadvantage is however compensated by low weight, available damping property, simple manufacture and clean handling (no rust, no lubrication). An optimal material use is obtained by a pressure and bending load. Springs of rubber principally have similar properties as those of synthetically produced materials, however their power ability is significantly lower. Springs of foamed material require a great structural volume, Springs of massive material require smaller space.
The most known springs of massive material are tubular springs of polyurethane, such as for example VULCOLLAN of BayerAG, and springs of copolymers of polyester. The relatively thick-walled tubular springs which are disclosed for example in the German patent document DE-11 47 255 are loaded only to up to 40% of their initial height so as not to overstretch the material, and with increasing deflection the force is constantly increased. The springs of copolymers of polyester also known from German patent documents DE 28 44 486 and DE-35 33 435 which correspond to U.S. Pat. Nos. 4,198,037 and 4,566,678 must be pre-pressed initially to 60-90% of their initial height, since the material of nature does not have high spring properties. For this so-called stretching of fibers, a press must be provided, and moreover, by prepressing some of the original structural height is lost. The same is true for the springs disclosed in the German patent document DE 197 00 629 which corresponds to U.S. Pat. No. 5,791,637, which is however expensive with respect to its shape and manufacture.
For many applications, a short structural shape with the spring with high work take-up and low counterforce at high damping and good repeatability is required, especially as a price valuable replacement for less loadable (less than one loading per hour) hydraulic dampers. With the known synthetic plastic springs this can be achieved by overloading. The disadvantage is however a substantial reduction of the service life and a strong settling of the spring.
Accordingly, it is an object of the present invention to provide a compact spring element with damping properties, which fulfils the above mentioned requirements and simultaneously avoids the disadvantages of the prior art.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in spring element of an elastic material, in particular of synthetic plastic for taking a kinetic energy, which has a tubular piece, and a base plate which is fixedly connected to the tubular piece, the tubular piece having a wall thickness corresponding to a ratio between an outer diameter and an inner diameter of less than 2 and a structural height within the range of 0.7-1.3 of the outer diameter, so that during deflection with 35% of an initial structural height an open end of the tubular piece is fitted over a force-loaded surface.
In accordance with the present invention, the tubular piece of preferably massive polyurethane (one of nature of springy material with sufficient high damping) is fixedly connected with a base plate of preferably the same material. The tubular piece must have a wall thickness which corresponds to a ratio outer diameter to an inner diameter of less than 2, and a structural height in the region of 0.7-1.3 of the outer diameter. Preferably, the base plate has centrally an opening for a simple mounting of the spring element with a connecting means, such as a screw, on a horizontal or a vertical surface of a component to be protected from overloading.
With the thusly designed and mounted spring element, the end surface at the open end of the tubular piece can be acted upon by a counter surface with a force. The spring element can be compressed to 60% of its initial structural height, without overstretching the material. The spring element goes through three phases which are illustrated in FIGS. 3 and 5.
In phase 1 the tubular piece which is fixed with the ground plate is axially upset and extends radially outwardly in its central region.
In phase 2 with the deflection of approximately 35% which corresponds to an outer appearance image at which a part of the tubular wall is deformed outwardly at an angle of approximately 45xc2x0 to a force-introducing countersurface, the end surface of the tubular piece starts folding inwardly. This process takes place from inner form constraints and requires no force from outside. The spring element assumes a favorable form which is free from stresses. Thereby the tubular piece of the spring element at a reduced force can be further compressed without overstretching of the material.
In phase 3 with a deflection of approximately 55% with respect to the initial height, the force starts again increasing since the surfaces located orthogonal to the force application, in particular the inner surfaces formed by the radial extension, contact more and also the bending forces again increase. At approximately 50% deflection, the forces or stresses are reached as at 35% deflection. A further increase to 60% is not damaging since the force distribution is of large-surface type and results partially from the higher bending.
When the force is removed, without setting phenomena, the original shape and height of the spring element is restored. After maintaining a stationary phase which is required for all elastomeric springs, the loading process, which includes if necessary all remaining phases, is reproducible with the same values.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.